The “INESA Lighting Handbook,” ninth edition, published by the Illuminating Engineering Society of North America, is incorporated by reference here in its entirety. As discussed in chapter seven of that book, a “luminaire” is a device for producing, controlling, and distributing light. It is typically a complete lighting unit consisting of one or more lamps, sockets for positioning and protecting the lamps and for connecting the lamps to a supply of electric power, optical devices for distributing the light, and mechanical components for supporting or attaching the luminaire. Luminaires are also sometimes referred to as “light fixtures.”
Luminaires are usually classified by their application, such as residential, commercial, or industrial. However, a particular luminaire can often be used in more than one application, depending upon its performance characteristics. For example, so-called “high bay” and “low bay” luminaires are often used for general lighting in industrial and other settings. Low bay luminaires are generally designed to provide adequate vertical illumination for spaces with mounting heights that are less than about 20 feet. High bay luminaires, on the other hand, are generally used in applications with mounting heights that are greater than 20 feet. However, high bay luminaires can also be used at lower mounting heights to produce light distributions that vary from narrow to wide, depending on the application and the need for vertical illumination.
For high bay, low bay, and other luminaires, the light distribution is often controlled using a “refractor.” Refractors are light control devices that take advantage of the change in direction that light undergoes as it passes through the boundary of materials having different optical densities (or indices of refraction), such as air to glass or air to plastic. This redirecting is typically accomplished with two-or three-dimensional prisms that are raised from, or embossed into, the surface of a translucent material, such as acrylic plastic, polycarbonate, or glass. When the prisms are formed on the surface of a substantially flat sheet of material, then the sheet is sometimes referred to as a “prismatic lens.”
The material that is used in a refractor may also have reflective characteristics, such as those that produce a phenomenon known as “total internal reflection.” In this phenomenon, light passes through the first surface of the refractive material and is mostly reflected from the second surface, back into the material, and out the first surface. Due to this and other reflective properties of many light-transmitting materials, the term “reflector” is sometimes loosely interchanged with the term “refractor” in connection with light distribution devices in luminaires.
Luminaire performance is typically described in terms of a combination of electrical, photometric, mechanical, thermal, and/or other characteristics. Photometric performance refers to the efficiency and effectiveness with which a luminaire delivers light to an intended target. The mechanical performance of a luminaire refers to its behavior under environmental extremes such as water spray, moisture, or dust, while thermal performance describes the behavior of the luminaire at elevated temperatures.
Photometric performance is often described in terms of various light distribution characteristics of a luminaire. For example, a “luminous intensity distribution curve” may be used to represent the variation of luminous intensity in a plane through the light center of the luminaire. (The light center is the center of the smallest sphere that would completely contain the light-omitting element of the lamp, often simply the center of the arc tube.) Indoor luminaires are typically described in terms of a vertical intensity distribution curve that is obtained by taking measurements at various angles of elevation about a source in a vertical plane through the light center. Unless otherwise specified, the vertical distribution curve is assumed to represent an average such as would be obtained by rotating the luminaire about its vertical axis where the origin, or “nadir,” is downward through the light center.
Luminous intensity values are typically recorded at various vertical elevations between 0° to 90° or 180°. The total number of lumens emitted by the luminaire can then be calculated from the luminous intensity distribution. Dividing this total number of lumens produced by the luminaire by the number of lumens that are omitted by the lamp then provides the overall efficiency of the luminaire. Luminous intensity can also be determined for nested, solid angle, cones having apexes at the luminaire photometric center. Each cone then defines a conic zone and the lumens within each zone are referred to as “zonal lumens.” This zonal lumen data is often presented for each zone as a percentage of the total lumens produced by the fixture (“% Fixture” or “% F”) and/or a percentage of the total lumens produced by the lamp, without the fixture (“% Lamp” or “% L”).
Photometric performance can also be described in terms of a luminaire spacing criterion, or “SC.” The SC of a luminaire is an estimated maximum ratio of spacing to mounting height above the work plane for a regular array of luminaires such that the work plane illumination will be acceptably uniform. SC is often loosely used interchangeably with spacing-to-mounting-height ratio, or “SMH” which is now a disfavored term. For luminaires with an adjustable SC, it is generally preferred that the beam width be adjustable over as broad a range as possible.
The mechanical and thermal performance characteristics of a luminaire can also be described in a variety of ways. For example, as noted above, certain applications require luminaires which are resistant to liquid and/or vapor infiltration. With regard to thermal performance, the effect of lamp heat on the luminaire materials can also be quite important because various refractor materials exhibit poor heat resistance above certain temperatures.
For example, although acrylic refractors can be formed using a wide variety of common manufacturing techniques, they generally have poor resistance to heat when used in lighting applications at temperatures above 90° C. Consequently, the maximum lamp wattage for a particular luminaire will often be limited by the temperature at which the acrylic refractor starts to discolor. Although so-called “high heat acrylics” or polycarbonate materials can sometimes be used to allow for higher-wattage lamps, they are generally more-expensive and can only withstand about an additional 20-50° C. increase in continuous operating temperature.
Cooper Lighting of Peachtree City, Ga. offers a wide variety of industrial luminaires under its LUMARK® brand. For example, Cooper's SS series of prismatic high-bay products and FP series of prismatic food processing luminaires are provided with acrylic prismatic refractors and have multiple field-adjustable lamp positions for providing various light-distribution patterns. The beam dispersion of these luminaires can be increased, or decreased, by positioning the lamp closer, or further, from the opening of the refractor. Cooper Lighting's SS, FP and HB Series of products are optionally provided with a prismatic drop lens for covering the opening of the refractor. Cooper Lighting and/or its parent, Cooper Industries, Inc. also own a variety of patents covering various aspects of industrial lighting including U.S. Pat. No. 4,403,277 which is incorporated by reference here in its entirety.
LexaLite International Corporation of Charlevoix, Mich. offers a line of acrylic and polycarbonate refractors that it refers to as its “800 Series Prismatic Reflexor®.” For example, information about LexaLite's Model 822 is available at www.lexalite.com/822.html, including images, zonal lumen, and other data. Conical lenses and drop lenses are also available for Lexalite's Model 822. These and other of LexaLite products are allegedly covered by U.S. Pat. Nos. 4,839,781, 5,446,606, and D367,337, each of which is also incorporated by reference here in its entirety.
U.S. Pat. No. 4,839,781 to Barnes et al. discloses a reflector/refractor device that is generally shaped as an inverted bowl. The body of the device is defined by a series of sectional zones that are formed as frustro-toroidal segments. The outside surface of the body is formed with a plurality of reflective/refractive prism elements that consist of curved and angled surfaces. Internal rays impinging on these surfaces will be reflected or refracted as the incident angle is greater than or less than the critical angle of the transparent material forming the body.
Each of the Barnes et al. zones has a predetermined radius with an origin that is offset from the vertical axis in order to create the bowl-shaped profile of the body. The light distribution characteristics for the top and bottom zones are selectively variable by vertical movement of the light source so as to increase or decrease the incident angle to the inner surface of the body. This, in turn, increases or decreases the internal incident angle to the corresponding prism element with respect to the critical angle of the material so as to reflect or transmit, through refraction, individual rays.
U.S. Pat. No. 5,444,606, also to Barnes et al., discloses a combination of a prismatic reflector and a prismatic lens for use with lighting fixtures. The reflector body has a substantially parabolic contour defining an interior cavity and includes a plurality of prisms for receiving, transmitting, and reflecting light. An inside surface of the reflector includes a smooth light-receiving, lower surface portion and an optional light depressing prism portion. The prisms provide modest or slight light spreading for additional rays near nadir. Rays which are omitted from the lamp to near the top, middle, and bottom of the reflector are illustrated as striking the surface at near normal angles in order to reduce first surface reflections and refraction losses.
U.S. Pat. No. 4,903,180 to Taylor et al. (assigned at issuance to General Electric Company) is also incorporated by reference here and discloses a luminaire with a protected prismatic reflector. The dome-shaped reflector includes a series of superimposed integrally-connected sections, each of a truncated conical form that tapers to a progressively greater extent than the one beneath it. This patent also notes that a substantial amount of light will pass through the reflector even though the reflecting surfaces of the prisms are clean. According to the patent, one reason for this effect is that the molds used for making such reflectors are not precise enough to achieve mathematical precision of the reflecting surfaces all the way to the apexes of the prisms and to the nadir of the valleys between them. Additional light leakage can also occur at points of defects in the prism surfaces.